1. Field of the Invention
The present invention relates in general to an in-plane switching liquid crystal display (IPS-LCD), and in particular to electrode structures of an IPS-LCD.
2. Description of the Related Art
Liquid crystal displays (LCDs) are classified by the orientation of the LC molecules interposed between the glass substrates. In a conventional twisted nematic LCD (TN-LCD), the LC molecules are twisted between the two substrates. In contrast, in an in-plane switching LCD (IPS-LCD), common electrodes and pixel electrodes are formed on a lower glass substrate (TFT substrate) and an in-plane electric field therebetween is generated for rearranging the LC molecules along the electric field. Accordingly, the IPS-LCD has been used or suggested for improving drawbacks of the conventional TN-LCD, such as a very narrow viewing angle and a low contrast ratio.
In order to achieve better performance of the in-plane electric field, a comb-shaped electrode array is built into the IPS-LCD to solve problems such as an insufficient aperture ratio and crosstalk produced between data lines and common electrodes. FIGS. 1A and 1B are sectional diagrams of a conventional IPS-LCD. FIG. 1C is a top view of the electrode structures of a conventional IPS-LCD. FIG. 1A shows the alignment of the LC molecules in an off state, and FIG. 1B shows the alignment of the LC molecules at an on state. The IPS-LCD has a lower glass substrate 10, an upper glass substrate 12, and a liquid crystal layer 14 interposed between the two parallel glass substrates 10 and 12. A plurality of strip-shaped common electrodes 16 arranged as a comb-shape structure is patterned on the lower glass substrate 10 serving as a TFT substrate, an insulating layer 18 is deposited on the common electrodes 16 and the lower glass substrate 10, and a plurality of strip-shaped pixel electrodes 20 arranged as a comb-shape structure is patterned on the insulating layer 18.
As shown in FIG. 1A, before an external voltage is applied to the IPS-LCD, the LC molecules 14A are aligned in a direction parallel to the lower glass substrate 10. As shown in FIG. 1B, when an external voltage is applied to the IPS-LCD, an in-plain electric field 22 is generated between the common electrode 16 and the pixel electrode 20, resulting in rotation of the LC molecules 14B toward the in-plane electric field 22.
Depending on the material and the structural design of the common electrode 16 and the pixel electrode 20, the conventional comb-shaped electrode array is classified as three types. FIGS. 2A to 2C are sectional diagrams showing the three types of the common electrode 16 and the pixel electrode 20 in the conventional comb-shaped electrode array. In the first type, as shown in FIG. 2A, the common electrode 16 and the pixel electrode 20 are patterned on the same plane and made of a transparent conductive material, such as ITO or IZO. In the second type, as shown in FIG. 2B, the common electrode 16 made of a non-transparent conductive material, such as Al and MoW, is patterned on the lower glass substrate 10 and followed by depositing the insulating layer 18. The pixel electrode 20 made of a transparent conductive material, such as ITO or IZO, is then patterned on the insulating layer 18. In the third type, as shown in FIG. 2C, the common electrode 16 and the pixel electrode 20 are patterned on the same plane and made of a non-transparent conductive material, such as Al and MoW. In practice, however, the asymmetrical electrode structure of the IPS-LCD generates image sticking and flicker problems in IPS-LCD, typically referred to the flexoelectric effect.